1. Field of the Invention
The present invention relates to an oxygen sensor which detects an oxygen concentration or the presence or absence of oxygen in a gas component of a gas to be monitored, and more particularly to a solid electrolyte having an improved durability.
2. Description of the Prior Art
Heretofore, as taught in the U.S. Pat. No. 3,759,232 and the SAE Paper No. 750,368, published in February, 1975, it has been proposed to use an oxygen sensor to detect oxygen concentration in exhaust gas exhausted from an internal combustion engine in order to control the air-fuel ratio of a mixture to be supplied to the internal combustion engine. The oxygen sensor has a solid electrolyte which is made of an oxygen ion conducting metal oxide or solid electrolyte which generates an e.m.f. corresponding to a difference between an oxygen concentration or an oxygen partial pressure of a gas component of a gas to be monitored and an oxygen concentration or an oxygen partial pressure of a reference gas (e.g. atmosphere). On that surface of the solid electrolyte which is subjected to the reference gas, a thin film platinum electrode is formed by chemical plating, vacuum deposition or the like, and on that surface of the solid electrolyte which is subjected to the gas to be monitored, a thin film platinum electrode is also formed so that the e.m.fs generated at both electrodes of the solid electrolyte are taken.
However, the oxygen sensor of the type described above includes a disadvantage that extended use thereof results in deterioration of performance such that a sufficient e.m.f. to allow precise control of the air-fuel ratio cannot be taken.
After extensive study on the above problem, the inventors of the present invention have found that the cause of the above problem was that the thin film electrode on that surface of the solid electrolyte which was subjected to the gas to be monitored was exposed to that gas whenever the detector was used, and when the temperature variation of the gas was remarkable such as in the case of exhaust gas exhausted from the internal combustion engine, the electrode was consumed and peeled away in a short time because it was a thin film. Discussing the problem in further detail, in addition to the thin film structure of the electrode, the fact that the electrode and the solid electrolyte are made of different materials and the adherence therebetween is poor has a significant affect. The consumption and the peel-off of the electrode occur e.g. at the center of the electrode and they result in non-conduction among non-consumed or non-peeled portions of the electrode. As a result, a sufficient e.m.f. cannot be taken out.